Electrostatic coating system



Sept. 19, 1967 M. A. R. POINT 3,342,415

ELECTROSTATIC COATING SYSTEM Filed Feb. 18, 1964 2 SheetsSheet 1 Sept. 19, 1967 M. A. R. POINT ELECTROSTATIC COATING SYSTEM Filed Feb. 18, 1964 Fig-3 I "'m Trill/01111011111111).

2 Sheets-Sheet 2 United States Patent 3,342,415 ELECTROSTATIC COATING SYSTEM Marcel A. R. Point, Grenoble, Isere, France, assignor to SAMES-Societe Anonyme de Machines Electrostatiques, Paris, France, a French joint-stock company Filed Feb. 18, 1964, Ser. No. 345,723 Claims priority, application France, Feb. 19, 1963, 925,214, Patent 1,361,917 4 Claims. (Cl. 239-) This invention relates to coating systems and more par ticularly to such systems in which the coating substances are applied to the articles to be coated through the agency of an electrostatic field.

Electrostatic coating systems of this general class have gained wide acceptance in recent years owing to the outstanding advantages they present over more conventional systems which involve spraying in the absence of an electric field. These advantages include more positive guidance of the particles of coating substance toward and onto the surfaces of the work being coated, whereby loss of coating substance is reduced to a minimum; creation of a strong electrostatic bond between the work surfaces and the coating substance, whereby subsequent heating or other bonding treatments can in many cases be dispensed with; more uniform coating thickness; and other advantages. A wide variety of coating substances may be eifectively employed in systems of this type. Representative coating substances include those which are applied in solid divided fomi, that is to say pulverulent substances such as enamels, plastic compositions and the like, and those which exist in liquid divided form, such as paints, etc.

In one common type of electrostatic coating system presently in use, there is provided a spray nozzle or gun to which the divided coating substance is delivered from a store thereof by means of a high velocity air stream. An electrode associated with or forming part of the nozzle is positioned so that the particles of coating substance suspended in the air stream must pass over or close to the electrode as they issue from the nozzle outlet. The electrode is connected to a high DC. potential, illustratively of the order of one hundred or more kilovolts, which is supplied by an electrostatic generator or other suitable source. The particles are thus charged to a corresponding potential and are positively guided over the lines of force of the electrostatic field created between the electrode and the surface of the work, which is usually at ground potential. The particles are bonded to the work surface by electrostatic attraction.

In a modification of this system which is especially useful where the coating substance is a liquid, e.g. paint, the liquid is delivered to the surface of a rapidly revolving disk provided in the spray gun. The disk discharges the liquid outwardly be centrifugal force from its periphery in the form of finely divided droplets which are then taken up by an axially flowing air stream. The revolving disk in some cases serves as the high voltage electrode for charging the droplets to the required potential.

Another known arrangement, as disclosed, for example, in Barford et al. US. application Ser. No. 287,554, now Patent No. 3,248,253. employs a relatively stationary, fluidized bed of coating powder. Air under pressure is injected upwardly through the perforate base of a tank containing the powder, and point electrodes connected to a high DC. potential project into the tank to charge the fluidized powder particles. The articles to be coated, preferably at ground potential, are brought into position in or just above the tank such that their surfaces attract the fluidized particles. The particles are bonded to the surfaces by electrostatic attraction.

The v-airous systems just outlined perform satisfactorily, but all involve the use of air or other fluid under pressure in comparatively large amounts, thereby complicating the installations.

In another known system, the coating powder is contained in an overhead tank having a perforate bottom and a point electrode projecting downwardly therefrom. The tank is vibrated to discharge the particles through the perforate bottom and past the electrode, thus charging the particles and causing the charged particles to travel under the combined actions of gravity and the electrostatic field onto the surfaces of the work below the tank. For a more detailed discussion of representative systems of this type, reference may be had, for example, to British Patent 851,685 and corresponding US. Patent 3,058,443.

This last arrangement is advantageous in that it dispenses with the use of pressurized air. However, the need for vibrating the tank introduces another complication. Moreover, the satisfactory operation of the system depends to some extent on a somewhat critical granulometry of the coating powder as well as on a precise adjustment of the height of the tank above the work, to ensure that the particles follow the prescribed path under the resultant of gravity and electrostatic forces. In addition, the system is not readily adaptable for use with liquid coating substances.

One general object of this invention, therefore, is to provide a new and improved system for applying a coating substance to the work through the agency of an electrostatic field.

More specifically, it is an object of the present invention to provide an electrostatic coating system which enables the realization of a smooth and uniform discharge of the coating substance without the need for a supply of air under pressure.

Another object of the invention is to provide a coating system of the character indicated in which solid power coating substances and 'liquid coating substances are applied to the work with substantially equal facility.

A further object of the invention is to provide such coating system which ensures the smooth discharge of a wide variety of coating substances having widely varying granulometric ranges.

Still another object of the invention is to provide an electrostatic coating system in which the coating is applied to objects of comparatively intricate shapes without the need for delicate vertical adjustments of the coating apparatus.

A still further object of the invention is to provide an electrostatic coating system which is particularly well suited for the simultaneous discharge of a number of different coating substances, including both solid and liquid substances. 1

Another object of the invention is to prOvide an electrostatic coating system which is economical to manufacture and thoroughly reliable in operation.

, In several advantageous embodiments of the invention, there is provided an electrostatic coating system which utilizes the simultaneous and combined actions of gravity, centrifugal force and electrostatic force to eifect in a simple and economical manner a smooth and positive deposition of a finely divided coating substance on the surfaces of the work. In these embodiments, there is provided an overhead store of the coating substance, a disklike discharge member driven in rapid rotation about a generally vertical axis and positioned below the store and adjacent the surface of an article to be coated, a feed passage leading downwardly from the store to a point overlying an upper surface of the disk-like member, so as to feed the substance thereto by gravity under a substantially static pressure head for discharge by cen- 3 trifugal force from the periphery of the member toward the surface of the article, and electrode means associated with or forming part of the periphery of the disk for charging the particles as are discharged therefrom and creating an electrostatic field for guiding the charged particles toward the article.

Exemplary embodiments of the invention will now be described, for purposes of illustration but not of limitation, with reference to the accompanying drawings, wherein:

FIGURE 1 is a simplified, partially schematic overhead view of portions of an electrostatic coating system according to one illustrative embodiment of the invention, as used in the enamelling of the internal surface of a bathtub;

FIGURE 2 is a diagrammatic view, with certain parts shown in section and others in elevation, of an electrostatic coating system in accordance with the embodiment of FIGURE 1; and

FIGURE 3 is an enlarged fragmentary sectional view, partly schematic, of an electrostatic coating system in accordance with another illustrative embodiment of the invention.

Referring to FIGURES l and 2, there is illustrated a bathtub 1 which is to have its inner concave surface coated with an enamelling composition. The bathtub 1 is made of an electrically conductive material, for instance cast iron, and is electrically connected to ground. An overhead monorail conveyor 2 extending above the bathtub 1 has a trolley 4 horizontally movable thereover by means of rollers 3. The trolley 4 supports a frame shown schematically at 12 which is arranged for vertical traversing movement relative to the trolley along guide rails 5. These guide rails extend in a downward direction from the trolley 4 and are provided with rollers 6 which engage the adjacent faces of the frame 12.

A tubular arm 7 projects downwardly from the frame 12. The arm 7 accommodates a vertical shaft 8 which is rotatably supported therein through suitable bearings (not visible in FIGURES l and 2) Secured to the projecting lower end of shaft 8 is a generally disk-shaped discharge member 10. The disk member 10 is preferably made of conductive material and is provided with a comparatively sharp peripheral edge. The member 10 includes a circumferential series of vertically extending holes 11 formed through it near its periphery. The upper end of the shaft 8 is coupled to an electric motor (not shown) carried by the frame 12.

The disk member 10 is electrically connected, as by a slip ring connection (not shown), to the negative terminal of a source of high D.C. voltage, the other terminal of which is grounded. The source 15 illustratively comprises an electrostatic generator having an output voltage of the order of one hundred or more kilovolts.

Supported by the frame 12 is a hopper 9 which contains a store 14 of coating substance. The coating substance may be in either solid or liquid form and illustratively comprises a suitable enamelling powder. The bottom outlet of the hopper 9 is connected to a vertical tube 13 which extends downwardly therefrom and terminates a short distance above the upper surface of disk member 10. Suitable valve means (not shown) are provided for controlling the discharge of the coating substance from the hopper 9 through the pipe 13.

In operation, power is applied to rotate the shaft 8 and the disk member 10 at high speed, and a high D.C. voltage is applied to the member 10 from the generator 15. The coating substance 14 flows by gravity from the hopper 9 down the pipe 13. Because of the substantial vertical length of the pipe and the consequent pressure head, the substance reaches the upper surface of the member 10 under substantial pressure. As the substance drops onto the member 10, it is projected outwardly by centrifugal force and travels over the upper surface of the member. A portion of the substance passes through the holes 11, while the remainder moves past the sharp peripheral edge of the member. During such travel, any small masses or caked fragments that may be present in the coating substance, if a powder, are broken up by centrifugal force and are discharged in substantially fully divided form. If the coating substance is a liquid, it similarly is broken up into minute droplets as it streams in rivulets over the upper surface of the member 1th The powder particles or liquid droplets, as the case may be, upon moving past the sharp electrode formed by the peripheral edge of the discharge member 10, become charged to a high D.C. potential. The thus charged material follows the lines of force, somewhat as indicated by the arrows in FIGURE 2, of the electrostatic field created between the member 10 and the inner surfaces of the bathtub 1. The portion of the coating substance which passes through the holes 11 for the most part is directed toward the lower, substantially horizontal surface of the bathtub, while the remaining portion moves toward the inner side walls of the tub. If desired, the frame 12 is continuously or intermittently re ciprocated horizontally over the monorail 2 and/or vet tically along the rails 5 along the length and/or depth of the tub. With this arrangement, the uniformity of the coating on the tub surfaces is further enhanced.

In the modified embodiment shown in FIGURE 3, parts corresponding in function to parts present in the first embodiment are designated with similar reference numerals, with an added prime The system of FIG= URE 3 comprises a hollow arm 7' depending from a frame which illustratively may be of the type shown at 12 in FIGURE 2, although the frame of the FIGURE 3 embodiment need not be arranged for vertical traverse movement relative to its supporting trolley. A tubular shaft 8 is supported for rotation within the arm 7 by a bearing structure 21 mounted in the lower end of the arm and a similar bearing structure (not shown) mounted in the upper arm end. The shaft 8 is provided with a pulley 22 secured thereto intermediate its ends. This pulley is connected by a belt 23 to a drive pulley 24 which is continuously rotated at a comparatively high speed. The belt 23 extends through an aperture 25 in the side wall of the arm 7.

The tubular shaft 8' extends beneath the lower end of the arm 7 and is provided with a series of vertically spaced disk members secured to it adjacent its lower end, the disk membersjn this example being three in number as shown at 10', 10'a and 10'b. Each of these disk members is fabricated from metal or other electrically conductive material and extends in a horizontal plane with relatively sharp peripheral edges. The lowermost disk member 10' is secured across the bottom end of the tubular shaft 8' and is formed with a circular array of apertures 11' therein. The other two disk members 10'a and 10b are secured around the tubular shaft 8 at suitably spaced points. A pair of upwardly curved annular flanges 27a and 27b are afiixed to the inner surface of the tubular shaft 8' adjacent the disk members 10a and 10'b, respectively. The shaft 8' is provided with sets of circumferentially oriented holes 2612 and 26b which permit communication between the upper surfaces of the respective disk members 10'a and 10'b and the corresponding surfaces of the flanges 27a and 27b. A similar set of holes 26 at the lower end of the shaft 8 permits communication between the inner and outer upper surfaces of the disk member 10'.

Extending longitudinally through the tubular shaft 8' are generally vertical pipes 13, 13a and 13b. The upper ends of these pipes are connected to one or more overhead tanks or hoppers containing the coating substance and supported from a trolley such as the trolley 4 in FIG- URE 2. The lower end of the pipe 13' is disposed a short distance above the inner part of the disk member 10, while the lower ends of the pipes 13a and 13'b are ori- 'ented short distances above the flanged inner parts 27a and 27b, respectively, of the disk members 'a and 10'b.

In operation, the pulley 24 is rotated at a comparatively high speed to similarly rotate the pulley 22 and the tubular shaft 8'. A high DC. potential is applied to each of the three disk members 10, 10a and 10'!) in a manner in general similar to that described heretofore with respect to the disk 10 of FIGURE 2. The coating substance or substances is discharged by gravity under a substantial pressure head through the conduits 13', 13a and 13b and is deposited on the upper internal surfaces of the disk member 10' and the flanges 27a and 27b. The coating material on these surfaces moves through the apertures 26, 26a and 26b to the outer portions of the disk members, with a portion of the material from the conduit '13 passing in a downward direction through the apertures 11 in the member 10. It is noted that the pipes 13a and 13b associated with the upper two disk members are each shown as being two in number, although this is not essential, and only one or more than two feed pipes per member may be used with good results. As the disk members rotate, the individual particles of coating substance thereon are electrically charged to a high DC. potential and are centrifugally discharged onto the surface being coated. The particles are attracted to the surface by the electrostatic field therebetween to provide an extremely smooth and uniform coating.

The provision of a plurality of vertically spaced disk members, such as the three members 10', ltla and 10b shown in FIGURE 3, for example, is useful in various ways. The arrangement is particularly well suited for coating the surface of an article having a relatively great depth or vertical extent, such as a bathtub, etc., without employing a vertical traverse of the discharge assembly. Such a multiple-disk arrangement also is advantageous in the simultaneous discharge of different coating substances or ingredients on a common work surface. For this purpose, the feed pipes 13', 13a and 13'!) are connected at their upper ends with stores of the different substances. The substances may comprise different types of coating powders and/or may include various liquid ingredients. As an example, in one arrangement two of the disk members serve to discharge a therm-osetting resin powder, while the third disk member acts simultaneously to discharge a setting agent or polymerization catalyst therefor. In these latter cases, it sometimes is advantageous to impart to the discharge assembly shown in FIG- ure 3 a vertical traverse movement similar to what was disclosed with reference to the first embodiment, to thereby improve the mixing of the various ingredients over the coated surface.

In both of the illustrated embodiments of the invention, the force of gravity and centrifugal force are uesd jointly with the force of the electrostatic field in order to accomplish a smooth and positive discharge of finely di vided coating particles, in solid and/ or liquid form, upon the surface of the work. Such a system is of particular value in connection with the coating of the internal surfaces of recessed articles, such as a bathtub, for example. The centrifugal force field created by the revolving discharge member or members inherently presents a symmetry of revolution about the axis of rotation, so that maximum efficiency is obtained in cases where the surface of the work substantially surrounds the discharge member. In some cases, the system is employed to simultaneously coat a plurality of articles, each of which includes a surface which is flat or of other shape, by arranging the articles in a circumferential annular array around the periphery of the discharge member. In other embodiments, the system is advantageous for applications where the work surfaces are not dis-posed around the discharge member, as by providing means for masking a portion of the circumference of the member with a shield or bafide, for example, or for other applications.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. An electrostatic coating system comprising, in combination, an overhead store of coating substance, a plurality of generally fiat rotary discharge members mounted below said store adjacent the surface of the work to be coated in spaced-apart relationship with each other, each of said discharge members having a sharp peripheral edge, means defining a feed passage leading downwardly from said store to a point overlying each of said discharge members, to feed the coating substance thereto by gravity, each of said members having an open, unobstructed upper face for receiving said coating substance from said passage defining means and for permitting the free discharge and dispersion of said substance in divided form by centrifugal force from the periphery of the corresponding member, a source of high DC. potential relative to the work to be coated, and means for supplying said potential to said discharge members to electrically charge the particles of said coating substance as they are discharged therefrom, to thereby produce an electrostatic attraction between said particles and the surface being coated.

2. An electrostatic coating system comprising, in combination, an overhead store of coating substance, a plurality of disk-shaped discharge members mounted below said store adjacent the surface of the work to be coated, each of said members being supported for rotation about the same vertical axis and having a sharp peripheral edge thereon, conduit means defining a plurality of feed passages, at least one of said passages being provided for each of said discharge members, each of said passages leading downwardly from said store to a point overlying the corresponding member to feed the coating substance thereto by gravity, each of said members having an open, unobstructed upper face for receiving said coating substance from the corresponding downwardly leading feed passage and for permitting the free discharge and disper sion of said substance in divided form by centrifugal force from the associated member, a source of high D.C. potential relative to the work to be coated, and means for supplying said potential to each of said discharge members to electrically charge the particles of said coating substance as they are discharged therefrom, to thereby pro duce an electrostatic attraction between said particles and the surface being coated.

3. An electrostatic coating system comprising, in combination, an overhead frame, a store of coating substance carried by said frame, a supporting structure having an elongated vertically extending shaft depending from said frame, means including a plurality of generally diskshaped discharge members mounted on the lower end of said shaft, each of said discharge members having a sharp peripheral edge and being arranged for rotary movement about the axis of said shaft adjacent the surface of the Work to be coated, said supporting structure defining a plurality of feed passages, one of said passages being provided for each of said discharge members, each of said feed passages leading downwardly from said store to a pomt overlying the corresponding discharge member, to feed said coating substance thereto by gravity under a substantial pressure head, each of said members having an open, unobstructed upper face for receiving said coating substance from the corresponding downwardly leading feed passage and for permitting the free discharge and dispersion of said substance in divided form by centrifugal force from the periphery of the associated member, a source of high DC. potential relative to the work, and means connected to said source for electrically charging the particles of said substance as they are discharged from the periphery of each of said discharge members, to thereby produce an electrostatic attraction between said particles and the surface being coated.

4. An electrostatic coating system comprising, in combination, an overhead frame, a store of coating substance carried by said frame, a supporting structure having a vertically extending tubular shaft depending from said frame, means including a plurality of generally diskshaped discharge members mounted on the lower end of said shaft, each of said discharge members having a central portion within said shaft and a sharp peripheral edge portion carried externally with respect to said shaft, said members being mounted for rotation about the axis of said shaft adjacent the surface of the work to be coated, said supporting structure defining a plurality of feed passages, at least one of said passages being provided for each of said discharge members, each of said feed passages leading downwardly through said shaft from said store to a point overlying the central portion of the corresponding discharge member, to feed said coating substance thereto by gravity under a substantial pressure head, said shaft including a series of openings adjacent each of said discharge members to permit the passage of said substance from the central portion thereof to its edge portion by centrifugal force, each said member having an open, unobstructed upper face for receiving said coating substance: from the corresponding downwardly leading feed passage: and for permitting the free discharge and dispersion of said substance in divided form by centrifugal force from the periphery of the associated member, a source of high DC. potential relative to the work, and means connected to said source for electrically charging the particles of said substance as they are discharged from the periphery of each of said discharge members, to thereby produce an electrostatic attraction between said particles and the surface being coated.

References Cited UNITED STATES PATENTS 2,893,893 7/ 1959 Crouse 23915 2,955,565 10/ 1960 Schotland 23915 2,975,756 3/1961 Reindl et al 239-15 3,085,749 4/1963 Schweitzer et al. 239-6 3,144,209 8/1964 Griffiths 23915 3,197,143 7/1965 Norris 239-45 EVERETT W. KIRBY, Primary Examiner. 

1. AN ELECTROSTATIC COATING SYSTEM COMPRISING, IN COMBINATION, AN OVERHEAD STORE OF COATING SUBSTANCE, A PLURALITY OF GENERALLY FLAT ROTARY DISCHARGE MEMBERS MOUNTED BELOW SAID STORE ADJACENT THE SURFACE OF THE WORK TO BE COATED IN SPACED-APART RELATIONSHIP WITH EACH OTHER, EACH OF SAID DISCHARGE MEMBERS HAVING A SHARP PERIPHERAL EDGE, MEANS DEFINING A FEED PASSAGE LEADING DOWNWARDLY FROM SAID STORE TO A POINT OVERLYING EACH OF SAID DISCHARGE MEMBERS, TO FEED THE COATING SUBSTANCE THERETO BY GRAVITY, EACH OF SAID MEMBERS HAVING AN OPEN, UNOBSTRUCTED UPPER FACE FOR RECEIVING SAID COATING SUBSTANCE FROM SAID PASSAGE DEFINING MEANS AND FOR PERMITTING THE FREE DISCHARGE AND DISPERSION OF SAID SUBSTANCE IN DIVIDED FORM BY CENTRIFUGAL FORCE FROM THE PERIPHERY OF THE CORRESPONDING MEMBER, A SOURCE OF HIGH D.C. POTENTIAL RELATIVE TO THE WORK TO BE COATED, AND MEANS FOR SUPPLYING SAID POTENTIAL TO SAID DISCHARGE MEMBERS TO ELECTRICALLY CHARGE THE PARTICLES OF SAID COATING SUBSTANCE AS THEY ARE DISCHARGED THEREFROM, TO THEREBY PRODUCE AN ELECTROSTATIC ATTRACTION BETWEEN SAID PARTICLES AND THE SURFACE BEING COATED. 